(1) Field of the Invention
The present invention relates to an image forming apparatus with a transfer device which transfers the toner images formed on the image bearer to transfer media, and preferably relates to an image forming apparatus in which toner images are transferred to transfer media under the presence of electric field, such as a copier, laser beam printer and other image recording apparatus using an electrophotographic process including liquid development process.
(2) Description of the Prior Art
FIGS. 1A and 1B show a conventional electrophotographic image forming apparatus such as a copier, laser printer or the like. In FIG. 1A, the image forming apparatus includes: a photosensitive member 101 as an image bearer; a charger 102 for charging photosensitive member 101; an exposure unit 103 for forming a latent image by light exposure; a developing unit 104 for performing development with toner; a transfer device 105 for transferring the toner image to a transfer medium 108; a fixing device (not shown) for fixing the toner image on transfer medium 108; an erasing device 106 for erasing charge on the photosensitive member; and a cleaning unit 107 for removing the leftover toner from the photosensitive member. This apparatus further includes power supplies 109, 110 and 111 for applying voltages to charger 102, developing unit 104 and transfer device 105, respectively.
In recent years, various contact type transfer devices have been developed in order to provide an ozoneless, low-cost, compact and energy saving configuration for transfer device 105 for transferring the toner image on photosensitive member 101 to transfer medium 108, as shown in FIG. 1B.
This contact type transfer device 105 has a roller configuration made of a metal core 105-1 of aluminum, iron or the like, which is covered with an electrically conductive tubular elastomeric element 105-2 or an electrically insulative tubular elastomeric element 105-2 (polyurethane, EPDM, silicone rubber, NBR, etc.) in which a conductor (ionic conductors, carbon black, metal oxides, metal powders, graphite, etc.) is dispersed. This roller (to be referred to hereinbelow as transfer roller) is set in abutment with the photosensitive member 101 surface and a bias voltage of +(xe2x88x92)500 V to +(xe2x88x92)3000 V is applied to metal core 105-1, so as to cause the toner on photosensitive member 101 to transfer to transfer medium 108.
Other than the above problem, ozone generation due to a Paschen discharge from transfer device 105 should be mentioned. Contact type transfer device 105 has a roller configuration made of a metal core 105-1 of iron, aluminum or the like, which is covered with an elastomer(silicone rubber, polyurethane rubber, EPDM, NBR, etc.) 105-2 in which a conductor (such as ionic conductors, carbon black, metal oxides, metal powders, graphite, etc. as a conductive filler) is dispersed. A voltage is applied to the metal core when this roller is set in abutment with the photosensitive member 101 surface with transfer medium 108 in between so that the voltage can be applied to the undersurface of transfer medium 108. The volume resistivity of the elastomer used in this configuration is 106 to 1010 xcexa9xe2x80xa2cm.
However, since the conventional transfer device 105 performs a transfer operation by applying a bias voltage to transfer roller core 105-1, high-voltage power supply 111 is needed, which leads to increase in the cost of the apparatus, increase in apparatus size for installing the power source, increase in consumption of power and increase in the number of consumable parts, results in inconsistency with regard to energy saving and ecologically-oriented development, which have become increasingly important for manufactures.
It is true that the above conventional contact transfer device 105 thus configured generates ozone in an amount of only about {fraction (1/50)} as much as that from a corona-discharge type, but it still releases ozone.
The inventors hereof have earnestly studied the mechanism of ozone generation from the contact type transfer device. As a result, it was found that Paschen discharge will occur at the micro gap Ep (the gap distance of about 10 to 100 xcexcm) near the nip N on the entrance side with respect to the rotational direction of the photosensitive member, generating ozone. It has been known that the thus generated ozone corrodes the photosensitive member and other elements, degrading the image quality.
The present invention has been achieved in order to solve the above problems, it is therefore an object of the present invention to provide an image forming apparatus capable of transferring images from the photosensitive member to transfer media without applying a bias voltage to the transfer device and generating even a small amount of ozone.
The present invention for attaining the above object is configured as follows:
In accordance with the first aspect of the present invention, an image forming apparatus including a transfer device for transferring toner images from an image bearer to a transfer medium comprising:
a ferroelectric subjected to a dipole orienting treatment, and
the transfer device being arranged opposing to the image bearer and having a layer containing the ferroelectric at least as part,
wherein the toner images are transferred to the transfer medium by electric field formed by the dipoles thus oriented.
In accordance with the second aspect of the present invention, the image forming apparatus having the above first aspect is characterized in that the transfer device is set floating without any voltage applied thereto.
In accordance with the third aspect of the present invention, the image forming apparatus having the above first or second aspect is characterized in that the transfer device is constructed such that the ferroelectric layer is formed on an electrically conductive support.
In accordance with the fourth aspect of the present invention, the image forming apparatus having the above third aspect is characterized in that the electrically conductive support is grounded.
In accordance with the fifth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the polarity of the ferroelectric layer is set positive when the toner on the image bearer is charged negative and the polarity of the ferroelectric layer is set negative when the toner on the image bearer is charged positive.
In accordance with the sixth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that a potential difference is given between the surface potential of the ferroelectric and that of the toner portion on the image bearer.
In accordance with the seventh aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the ferroelectric layer is formed with a film thickness of 8 xcexcm or greater.
In accordance with the eighth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the ferroelectric at least includes an organic material as part thereof.
In accordance with the ninth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the ferroelectric at least includes an inorganic material as part thereof.
In accordance with the tenth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the inorganic material is a ceramics sintered compact composed of at least three components.
In accordance with the eleventh aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that an abrasive-resistant material covers or coats the surface layer of the ferroelectric.
In accordance with the twelfth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the relative permittivitty of the ferroelectric is set equal to or greater than 10.
In accordance with the thirteenth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the volume resistivity of the ferroelectric falls within the range from 1014 xcexa9xe2x80xa2cm to 1015 xcexa9xe2x80xa2cm.
In accordance with the fourteenth aspect of the present invention, the image forming apparatus having the above thirteenth aspect is characterized in that the volume resistivity of the ferroelectric is set to be equal to or lower than 1012 xcexa9xe2x80xa2cm when it is heated within the range below the Curie temperature.
In accordance with the fifteenth aspect of the present invention, the image forming apparatus having any one of the above first through fourteenth aspects further comprises a heater for heating the ferroelectric layer arranged close to or in abutment with the ferroelectric layer.
In accordance with the sixteenth aspect of the present invention, the image forming apparatus having any one of the above first through fifteenth aspects further comprises: a potential detector for detecting the surface potential of the transfer device.
In accordance with the seventeenth aspect of the present invention, the image forming apparatus having the above sixteenth aspect is characterized in that based on the detected signal from the potential detector, the ferroelectric is heated under control up to the Curie temperature by the heater.
In accordance with the eighteenth aspect of the present invention, the image forming apparatus having any one of the above first through fourteenth aspects further comprises an erasing portion for erasing the charge on the ferroelectric layer.
In accordance with the nineteenth aspect of the present invention, the image forming apparatus having the above eighteenth aspect is characterized in that the erasing portion is a conductive brush arranged in abutment with the transfer device.
In accordance with the twentieth aspect of the present invention, the image forming apparatus having the above eighteenth aspect is characterized in that the erasing portion is a conductive roller having a conductive surface arranged in abutment with the transfer device.
In accordance with the twenty-first aspect of the present invention, the image forming apparatus having any one of the above eighteenth through twentieth aspects is characterized in that the erasing portion is grounded.
In accordance with the twenty-second aspect of the present invention, the image forming apparatus having any one of the above first through twenty-first aspects is characterized in that the transfer device is provided in a roller configuration which is comprised of a metal core, an electrically conductive elastomer as the first coating layer formed on the metal core surface and a ferroelectric layer as the second coating layer formed on the first coating layer.
In accordance with the twenty-third aspect of the present invention, the image forming apparatus having any one of the above first through twenty-first aspects is characterized in that the transfer device is provided in a roller configuration which is comprised of a metal core and a ferroelectric layer as the first coating layer formed on the metal core surface.
In accordance with the twenty-fourth aspect of the present invention, the image forming apparatus having the above twenty-second or twenty-third aspect is characterized in that the metal core is an aluminum core and the surface thereof in contact with the ferroelectric layer is anodized.
In accordance with the twenty-fifth aspect of the present invention, the image forming apparatus having any one of the above twenty-second through twenty-fourth aspects is characterized in that the transfer device is configured so that an abrasive-resistant material covers or coats the surface layer of the ferroelectric.
In accordance with the twenty-sixth aspect of the present invention, the image forming apparatus having any one of the above twenty-second through twenty-fifth aspects is characterized in that the surface potential in the ferroelectric layer of the transfer device is uniformly electrified at a potential within the range from +200 V to +1600 V, by a dipole orienting treatment.
The present inventors hereof have earnestly studied and successfully provided a transfer device for transferring the toner image formed on the surface of the image bearer, i.e., photosensitive member, from the photosensitive member to transfer medium (such as paper, OHP sheet etc.), in which a dipole oriented (poled) layer, e.g., a ferroelectric layer in the present invention, is formed on the surface layer in abutment with the photosensitive member with the transfer medium in between so that the charged toner adhering to the photosensitive member is made to transfer to the transfer medium by the function of the electric field formed by the dipoles of the transfer roller of the present invention, to thereby provide a recorded image. Thus, the present inventors have successfully completed the invention of a transfer roller which is suitable for this novel transfer process.